17.2.1.2 Gastropods
Thirty-four gastropod species alien to South Africa have formal global EICAT
assessments. Four of these—Helisoma duryi (Seminole Rams-Horn), Tarebia
granifera (Quilted Melania Snail), Oxychilus draparnaudi (Drapanaud’s Glass
Snail) and Theba pisana (White Garden Snail)—are known to change community
structures in their global invaded range (Kesner and Kumschick 2018). For example,
H. duryi outcompetes and displaces native snail species (e.g. Christie et al. 1981), T.
granifera has been implicated in causing local extinctions of native snails in
wetlands (e.g. Karatayev et al. 2009) and O. draparnaudi is a predator that causes
the local disappearance of prey species (e.g. Frest and Sanders Rhodes 1982).
In South Africa, the only documented impacts are for Tarebia granifera and
Theba pisana. The former was accidentally introduced into South Africa through the
aquarium trade and has since invaded several rivers, lakes, wetlands and estuaries in
the eastern and northern parts of the country (Appleton et al. 2009). It can reach high
population densities, and has been implicated in displacing native snails and becoming the dominant component of invertebrate assemblages in invaded areas (Miranda
and Perissinotto 2014). Theba pisana can reach high densities on the west and south
coast of South Africa, with the potential to impact on native fauna and flora
(Odendaal et al. 2008).
17.2.1.3 Fish
No fish species have been globally assessed with EICAT to date, but an assessment
of alien fish species naturalised in South Africa identified five species that have
negative impacts (Marr et al. 2017). Micropterus dolomieu (Smallmouth Bass), M.
salmoides (Largemouth Bass), Oncorhynchus mykiss (Rainbow Trout), Salmo trutta
(Brown Trout) and Oreochromis niloticus (Nile Tilapia) are known to have adverse
impacts in South Africa that span all levels of biological organisation from gene to
ecosystem (Ellender and Weyl 2014). On a genetic level, the integrity of native
Oreochromis mossambicus (Mozambique Tilapia) is under threat from hybridisation
with O. niloticus, a species introduced for aquaculture (Firmat et al. 2013). Evidence
of species and population-level impacts are mainly linked to direct predation by O.
mykiss, S. trutta and Micropterus species (Black Basses), which have resulted in
local extirpations of native fishes and invertebrates (Ellender and Weyl 2014; van
der Walt et al. 2016; Shelton et al. 2018).
Oncorhynchus mykiss and S. trutta are cold water species that, as a result of
stocking for sport fishing, have naturalised in many cool, well-oxygenated headwater streams in the country (Ellender et al. 2017; Weyl et al. 2017a). They are
generalist predators that feed primarily on aquatic invertebrates, but will also
opportunistically prey on terrestrial invertebrates, fish and amphibians. Their
impacts therefore span numerous trophic levels, and in South Africa include the
decline, and in some cases local extirpation, of native invertebrates, frogs and fish
17 An Evaluation of the Impacts of Alien Species on Biodiversity
495
Thirty-four gastropod species alien to South Africa have formal global EICAT
assessments. Four of these—Helisoma duryi (Seminole Rams-Horn), Tarebia
granifera (Quilted Melania Snail), Oxychilus draparnaudi (Drapanaud’s Glass
Snail) and Theba pisana (White Garden Snail)—are known to change community
structures in their global invaded range (Kesner and Kumschick 2018). For example,
H. duryi outcompetes and displaces native snail species (e.g. Christie et al. 1981), T.
granifera has been implicated in causing local extinctions of native snails in
wetlands (e.g. Karatayev et al. 2009) and O. draparnaudi is a predator that causes
the local disappearance of prey species (e.g. Frest and Sanders Rhodes 1982).
In South Africa, the only documented impacts are for Tarebia granifera and
Theba pisana. The former was accidentally introduced into South Africa through the
aquarium trade and has since invaded several rivers, lakes, wetlands and estuaries in
the eastern and northern parts of the country (Appleton et al. 2009). It can reach high
population densities, and has been implicated in displacing native snails and becoming the dominant component of invertebrate assemblages in invaded areas (Miranda
and Perissinotto 2014). Theba pisana can reach high densities on the west and south
coast of South Africa, with the potential to impact on native fauna and flora
(Odendaal et al. 2008).
17.2.1.3 Fish
No fish species have been globally assessed with EICAT to date, but an assessment
of alien fish species naturalised in South Africa identified five species that have
negative impacts (Marr et al. 2017). Micropterus dolomieu (Smallmouth Bass), M.
salmoides (Largemouth Bass), Oncorhynchus mykiss (Rainbow Trout), Salmo trutta
(Brown Trout) and Oreochromis niloticus (Nile Tilapia) are known to have adverse
impacts in South Africa that span all levels of biological organisation from gene to
ecosystem (Ellender and Weyl 2014). On a genetic level, the integrity of native
Oreochromis mossambicus (Mozambique Tilapia) is under threat from hybridisation
with O. niloticus, a species introduced for aquaculture (Firmat et al. 2013). Evidence
of species and population-level impacts are mainly linked to direct predation by O.
mykiss, S. trutta and Micropterus species (Black Basses), which have resulted in
local extirpations of native fishes and invertebrates (Ellender and Weyl 2014; van
der Walt et al. 2016; Shelton et al. 2018).
Oncorhynchus mykiss and S. trutta are cold water species that, as a result of
stocking for sport fishing, have naturalised in many cool, well-oxygenated headwater streams in the country (Ellender et al. 2017; Weyl et al. 2017a). They are
generalist predators that feed primarily on aquatic invertebrates, but will also
opportunistically prey on terrestrial invertebrates, fish and amphibians. Their
impacts therefore span numerous trophic levels, and in South Africa include the
decline, and in some cases local extirpation, of native invertebrates, frogs and fish
17 An Evaluation of the Impacts of Alien Species on Biodiversity
495